LEDs (Light Emitting Diode) are semiconductor electronic components capable of emitting light, and such electronic components can only emit red light having a low-luminosity in the early stage. With the continuous improvements of technology, the electronic components have been developed to emit visible light, infrared light and ultraviolet light, and there are also great improvements in the luminosity. LEDs have advantages that cannot be incomparable by traditional light sources, such as high efficiency, long life, low damage possibility, high switching speed and high reliability, and have been widely used in fields of indicator lights, displays and lighting.
The power factor (PF) refers to the cosine of a phase difference between an input voltage and an input current. A low power factor will increase the loss of a power grid. Efficiency (EFF) is the ratio of a turn-on voltage of an LED to an input voltage, which satisfies the following relationship:
      Eff    =                  V        LED                    V        IN              ;low efficiency will increase the loss of a power supply. In general, in a linear LED driver, the PF value of its system is usually very high in the case of a high input voltage, but the efficiency is relatively low and the PF and the efficiency cannot be simultaneously taken into account.
FIG. 1 illustrates a common structure of a single-segment linear LED driver, wherein an AC voltage is converted into an input voltage VIN through a rectifier bridge, and supplies power for an LED light segment; the LED light segment consists of n LED lights connected in series; an output terminal of the LED segment is connected with a constant current control chip, and the constant current control is realized through the switching of a constant current control transistor in the constant current control chip; a capacitor C and a resistor R are connected in parallel with the two terminals of the input voltage to serve as adjustable devices. FIG. 2 illustrates an operating waveform of the above-mentioned single-segment linear LED driver structure. During different input voltage VIN periods, the higher the amplitude of the input voltage VIN is, the larger the power factor of the system will be. On the contrary, the higher the magnitude of the input voltage VIN is, the lower the efficiency of the system will be.
As shown in FIG. 3, a HIGH-VOLTAGE DROP CURRENT TECHNOLOGY during which the current should be reduced if the voltage is excessively high may generally be used to reduce an output current under a high input voltage, thus reducing the system loss and heat generation and improving the efficiency, but the power factor will decline in this case.
Therefore, the efficiency and the power factor are in inverse proportion. How to balance the efficiency and the power factor in the LED driving control and realize the compromise control of the efficiency and the power factor has become one of the problems to be urgently solved by those skilled in the art.